1. Field of the Invention
This invention relates to a wind turbine comprising:
a wind turbine tower with an upper end and a lower end, which lower end is provided on a wind turbine foundation;
a wind turbine nacelle provided at an upper end of said tower;
a hub provided at said nacelle;
at least two wind turbine rotor blades each with a mounting end and a free end, which mounting end is mounted on the hub for rotation in a rotational plane around an axis that is extended by shaft coupled to a generator or a gearbox;
grid connection for feeding produced electricity from the generator to a grid, which grid connection has voltage detection means for detecting changes in the voltage on the grid;
at least two masses each placed between the mounting end and the free end of at least two rotor blades.
2. Description of Related Art
This invention further relates to a method for controlling a wind turbine with a pitch system for pitching a blade in a pitch angle and with blades with a mass for increased inertia, which wind turbine is operated in a normal operation mode in which a generator has a generator speed at a generator torque, and which wind turbine is to remain electrically coupled to a grid during a low voltage condition and with supplied current specifications, torque reference, power reference, or according to a grid code; the method comprising the steps of:
detecting a low voltage condition with voltage detection means, which voltage detection means after detecting a low voltage condition send a request for:
a rotor de-rate procedure in a wind turbine controller; which de-rate procedure includes:
LVRT pitching the rotor blades to an LVRT pitch angle;
detecting a normal voltage condition with voltage detection means, which normal voltage condition is within a voltage range of the normal voltage condition; which voltage detection means after detecting a normal voltage condition send a request for:
normal operation mode of the wind turbine; which normal operation mode has an initial phase where:
the generator torque or power is increased to a desired torque reference or power reference, and
pitching the rotor blades from the LVRT pitch angle to a normal or freely controlled pitch angle.
In electric power generation and in particularly in the field of electric power generation by wind turbines, the electric power generation as a unit is required to interact or interface with a grid. This also means that the electric power generator is, and this is defined, required, requested or otherwise determined mainly by regulations, that the electric power generator is capable of adjusting or responding the changes on the grid.
In general the grid is defined as a transmission network that interconnects sources of electric power generators and sinks of electric power consumptions. The number of power sources, such as power stations, is often outnumbered by the number of power sinks, being individual households, commercial or industrial enterprises, public facilities or utilities.
In principle a grid can also be a so-called stand-alone-system with just one electric power generator as the source and just one electric power consuming unit as a sink.
Hence an electric power generator is a single unit that interacts with other sources or sinks also connected to the same grid.
The grid code sets requirements for grid connections of producers e.g. wind turbines for how to react to certain events on the grid.
As such the interaction between a power producing unit, or for that matter a power drawer unit, i.e. the connectees and the grid operator, is determined by a code determining and defining the grid. This code is the grid code.
Of technical importance are the voltage on the grid side and the voltage on the connectees side.
When the voltage on the grid side is normal, a wind turbine connected to the grid is designed to operate and be connected to the grid within a certain voltage range of the normal voltage condition. That is the wind turbine will rotate at a certain speed and be controlled to generate power within the range of the normal voltage conditions.
However, at times there is change of voltage on the grid. One such event is a so-called Low Voltage (LV) event. During such LV-event the wind turbine needs to be able to respond and act to stay connected to the grid.
Left uncontrolled, a LV-event will result in the rotor to increase its rotational speed; to over speed.
One way to reduce or eliminate the effect of a LV-event is to reduce the rotational speed of the rotor of the wind turbine to reduce or eliminate the over speed.
One solution is to brake the rotor mechanically to avoid over speed.
Another solution is to have an electric power generator system that includes a brake chopper, which brake chopper will simply burn energy in a dump, such as a resistor, thereby reducing the rotational speed of the rotor.
Brake choppers are know from patent applications, such as US 2007/0279815 A1 which corresponds to U.S. Pat. No. 7,586,216 B2, in which a brake chopper for de-energizing the generator in the wind turbine is disclosed.
Another example is disclosed in patent application WO 2010/085988, in which a method for allowing a wind turbine to remain electrically connected to a grid during a low voltage event is described. The method described relies on boosting a rotor current of the synchronous generator in response to the detected low voltage event.
The electrical brake system disclosed adds complexity and thereby requires extra resources during production and operation of the wind turbine.
Furthermore, the requirements of larger wind turbines increase the need for larger brake systems.
A person skilled in the art of making blades for a wind turbine has traditionally been occupied with making the blade lighter and stronger, whilst having the required flexibility. As the desire for making wind turbines even larger, the need for blades, that are even lighter, continues.
To such skilled person in the art, the optimal or ideal blade for a wind turbine is considered a shell and optimally just a skin shell forming a surface with the desired aerodynamically properties and the structural stability.
Likewise, the designer and producer of wind turbines will ideally ask for such ideal blade.
Over time blades for wind turbines have developed from being made of wood and with solid profiles, over blades made of glass fibres with hollow profiles towards carbon fibre blade structures.
Over the same time development systems and controls for operating a wind turbine with such optimised blades have been developed. Efforts to compensate for abnormal operation, including emergency operation have been devised as add-ons systems, elements usually placed in the hub, the tower or on the foundation or even adjacent from the wind turbine.
From International Patent Application Publication WO 2004/011801 A1 it is known to have movable masses arranged in the rotor blades in order to adjust the moment of inertia in relation to increase in wind speed. The solution mentioned in International Patent Application Publication WO 2004/011801 A1 concerns masses that are moved between different positions inside a wind turbine blade according to certain conditions as they appear. Moving said masses can be done in various ways with mechanical or electric motive elements.
WO 2004/011801 A1 does not teach how to control a wind turbine having blades with a length of e.g. 35 meters or more and a weight of e.g. 23000 kg or more during a low voltage ride through.
Instead it teaches how to increase the moment of inertia during high wind speeds by moving masses towards the tip of the blades and thus to accumulate kinetic energy in the rotor. Then during lower wind speeds the masses are moved towards the centre of the rotor in order to have a lower moment of inertia.
A solution as mentioned might be working when talking about rather small wind turbines with rather short and light wind turbine blades. In order to be able to change the moment of inertia in a modern wind turbine blade having a length of e.g. 35 meters or more and a weight of e.g. 23000 kg or more, it is quite a mass, for example several 1000 kilograms that need to be operated in each blade. Such a heavy mass is simply a problem to move between different positions in a blade according to the wind speed.
A similar solution is disclosed in U.S. Patent Application Publication US 2012/0107116 A1 which discloses a floating wind turbine with two moveable masses provided in each wind turbine blade in an elongated cavity. It teaches that the masses are designed to dampen the oscillating movement of the wind turbine structure and that the weight is determined based on the movement of the structure, such as 50 or 200 kilograms. Such a mass would not be suitable for changing the inertia of large and heavy modern wind turbine blade as mentioned above.